The load capacity of a foil thrust bearing depends on the compliance of the bearing with pressure exerted by a fluid film developed between the bearing and the runner. The pressure profile for a thrust bearing varies, and in order to accommodate the optimal pressure profile and attendant fluid film thickness associated with maximum load capacity, the thrust bearing should be designed to provide stiffness which varies in a manner similar to the pressure profile.
Current foil thrust bearings have limited load capacity. This limitation results from spring designs which indicate only a limited appreciation for variance in pressure profile and its effect on load capacity.
Typical spring designs are illustrated in U.S. Pat. No. 4,668,106 Gu. While such spring designs provide varying stiffness in radial directions, they provide limited load capacity because of excess pad deflection over the spring support points. The excessive pad deflection leads to a divergent fluid film at the trailing edge of the pad and prevents the bearing from developing an optimal pressure profile. Prior to the '839 application, a solution to this problem in the form of providing a spring design with varying stiffness in both radial and circumferential directions had gone unrecognized. The present invention is directed in one respect to spring designs which provide such variations in stiffness, and which offer both improved performance and greater facility in manufacturing.
A further complication arises in applications characterized by a circumferential pressure profile which exhibits a relatively gradual reduction near the trailing edge of the pad. It is believed that such a reduction will be more prevalent when the fluid is a liquid or high-pressure gas. In such applications, the spring design will be suboptimal to the extent that it does not provide for a commensurate reduction in stiffness near the trailing edge.
An objective of the invention is to provide spring cluster discs with stiffness variations which approximate pressure profile in both radial and circumferential directions.
A further objective is to provide spring cluster discs suitable for use in applications characterized by a relatively gradual reduction in circumferential pressure profile near the trailing edge of the foil pad.
A still further objective is to provide such discs with improved structural integrity. PG,4
Further objectives and advantages of the invention may be revealed by the following description, which includes the appended claims and accompanying drawings.